Many applications require the ability to sense acceleration and rate of rotation along three mutually orthogonal axes. In attempts to commercialize inertial measurement units for non-military applications, the cost and size of accelerometers have been addressed by various means. Sensing structures include capacitive devices as well as piezoresistive devices constructed using semiconductor manufacturing techniques. The capacitive devices are inherently less sensitive to temperature variations as they do not employ a resistance change which is highly sensitive to temperature. However, the electronics for a capacitive sensor are typically more complex.
Capacitive devices generally include a proof mass which is mounted on a substrate. Electrodes are positioned in directions of interest to sense the deflection induced by acceleration imposed on to the proof mass. These electrodes are located in the plane of the proof mass to provide X and Y detection as well as on the substrate surface to provide sensing in the Z direction or normal to the substrate surface. The deflection of the proof mass causes capacitances to change. This change is sensed by electronic circuits that provide a signal which is representative of the acceleration.
Performance of the accelerometer is driven by the fundamental frequency or modes of the structure and the sensitivity of the electronics. The movement to ever increasing miniaturization has imposed a greater challenge to the electronics. The ability to position and connect the electronics in close proximity with the position sensitive capacitors is critical to the performance of the circuit and consequently the performance of the accelerometer.
Micromachining of silicon has been utilized to produce both single axis and multiple axes sensors using compact form factors. A typical configuration includes a semiconductor layer interleaved between two highly insulating substrates such as glass. The glass does not introduce significant parasitic capacitances allowing the routing of the acceleration induced capacitance change to an external board electronics. This, however, poses some packaging challenges and consumes lateral space.
An approach is needed to satisfy the desire for a highly compact multiaxis accelerometer with good performance. An efficient utilization of space for both the mechanical sensing element as well as the electronics is desired. There are several challenges that must be met to realize a 3 axis accelerometer suitable for commercial inertial measurement applications. There are no known solutions to the problem of a 3 axis accelerometer compatible with a low cost multi-axes gyro. Many of these challenges have been met on an individual basis but not collectively in a single embodiment.
Compatibility with rate sensing sensors is an additional attraction on the path to an inertial measurement unit where six degrees of freedom are measured—3 orthogonal accelerations and 3 rate of rotation axes.
Accordingly, what is needed is a system and method for overcoming the above-identified problems. The present invention addresses such a need.